U.S. patent number 7,072,543 [Application Number 10/681,552] was granted by the patent office on 2006-07-04 for extended source transmitter for free space optical communication systems.
This patent grant is currently assigned to Terabeam Corporation. Invention is credited to John A. Bell, Carrie Sjaarda Cornish, Robert M. Pierce, David Rush.
United States Patent |
7,072,543 |
Pierce , et al. |
July 4, 2006 |
Extended source transmitter for free space optical communication
systems
Abstract
An apparatus for transmission of free space optical
communication system signals employing a spatially-extended light
source and method of using the same. A laser beam source directs an
optical signal into a free end of a segment of multimode fiber. As
the optical signal passes through the segment of multimode fiber,
the optical signal is converted into a mode-scrambled optical
signal. This mode-scrambled signal may then be used as a
spatially-extended light source that is directed outward as an
optical beam through the use of a collimating lens.
Inventors: |
Pierce; Robert M. (Longmont,
CO), Bell; John A. (Issaquah, WA), Cornish; Carrie
Sjaarda (Bellevue, WA), Rush; David (Sammamish, WA) |
Assignee: |
Terabeam Corporation (Falls
Church, VA)
|
Family
ID: |
29248527 |
Appl.
No.: |
10/681,552 |
Filed: |
October 7, 2003 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20040071398 A1 |
Apr 15, 2004 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
10128953 |
Apr 22, 2002 |
6810175 |
|
|
|
Current U.S.
Class: |
385/28; 385/33;
398/182; 398/200; 398/201 |
Current CPC
Class: |
G02B
6/14 (20130101); G02B 6/421 (20130101); G02B
6/4204 (20130101) |
Current International
Class: |
G02B
6/26 (20060101); G02B 6/42 (20060101) |
Field of
Search: |
;398/182,200,201
;385/28,33 ;372/6 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ullah; Akm Enayet
Assistant Examiner: Petkovsek; Daniel
Attorney, Agent or Firm: Blakely Sokoloff Taylor &
Zafman LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
The present application is a continuation-in-part of U.S. patent
application Ser. No. 10/128,953 filed Apr. 22, 2002, now U.S. Pat.
No. 6,810,175, entitled "Off-Axis Mode Scrambler" by Jun Shan Wey
et al., which is incorporated herein by reference. The present
invention is also related to U.S. patent application Ser. No.
09/886,248 filed Jun. 20, 2001, entitled "Multimode Optical Signal
Transmission in a Free-Space Optical Communication System" by Mark
Lewis Plett.
Claims
What is claimed is:
1. A free-space optical communication system (FSOCS) transmitter,
comprising: a spatially-extended light source including a laser to
generate a mode-scrambled optical signal, the spatially extended
light source further including a segment of multimode fiber having
a first end positioned to receive a laser optical signal produced
by the laser and a second end from which a mode-scrambled optical
signal is emitted and wherein a portion of the segment of multimode
fiber is configured in a series of alternating loops; a power
controller, operatively coupled to drive the laser; and output
optics, optically coupled to receive the mode-scrambled optical
signal from the spatially-extended light source and direct the
mode-scrambled optical signal outward from the FSOCS transmitter as
an optical beam having a controlled divergence.
2. The apparatus of claim 1, wherein the end of the segment of
multimode fiber is operatively coupled to the laser so as to
produce an offset-launched optical signal.
3. The apparatus of claim 1, wherein the segment of multimode fiber
consists of two or more separate pieces of multimode fiber that are
joined together to be a continuous single strand.
4. The apparatus of claim 3, wherein there are two joined segments
of multimode fiber wherein the first segment of multimode fiber
comprises a 62.5 micron graded-index core, and the second segment
of multimode fiber comprises a 200 micron step-index core.
5. The apparatus of claim 3, wherein the two or more segments of
multimode fibers are operatively coupled together using one or more
fusion splices.
6. The apparatus of claim 1, wherein the laser optical signal
converges or diverges at an angle that substantially matches a
numerical aperture of the multimode fiber.
7. The apparatus of claim 1, wherein the mode-scrambled optical
signal has a power intensity distribution that has a shape
substantially similar to a top hat.
8. The apparatus of claim 1, wherein an output of the
spatially-extended light source is located coincident with a focal
plane of a collimating lens comprising the output optics.
9. The apparatus of claim 1, further comprising a data modulator
operatively coupled to the spatially-extended light source, the
data modulator to modulate the optical beam.
10. The apparatus of claim 1, wherein the optical beam has a
wavelength from 400 to 1400 nanometers.
11. A method for generating a free space optical communication
system (FSOCS) signal, comprising: producing a mode-scrambled
modulated optical signal with a spatially-extended light source by:
operatively coupling a segment of multimode fiber configured in a
series of alternating loops; and directing an optical signal
produced by a laser to the first end of the segment of multimode
fiber to generate a launched optical signal that is received by the
first segment of multimode fiber; wherein, as the launched optical
signal passes through the segment of multimode fiber, it is
converted into a mode-scrambled optical signal that serves as a
spatially extended light source; passing the modulated optical
signal through a collimating lens to output an optical beam
comprising the FSOCS signal.
12. The method of claim 11, wherein the segment of multimode fiber
consists of two or more separate pieces of multimode fiber, with
possibly differing core sizes and index profiles, that have been
joined together to form a continuous single strand.
13. The method of claim 12, wherein there are two joined segments
of multimode fiber wherein the first segment of multimode fiber
comprises a 62.5 micron graded-index core, and the second segment
of multimode fiber comprises a 200 micron step-index core.
14. The method of claim 11, wherein the optical signal is directed
towards the first end of the segment of multimode fiber such that
it is received at an offset angle relative to a centerline of an
end portion of the segment of multimode fiber.
15. The method of claim 11, further comprising focusing the optical
signal into an end of the segment of multimode fiber such that the
optical signal is launched into the end at a point that is offset
from a centerline of the multimode fiber.
16. The method of claim 11, further comprising focusing the optical
signal such that it converges at an angle that substantially
matches a numerical aperture of the segment of multimode fiber.
17. A free-space optical communication system (FSOCS) transmitter,
comprising: means for generating a spatially-scrambled optical
signal that functions as an extended light source, wherein the
means for generating a spatially-scrambled optical signal comprises
an optical fiber segment configured in a series of alternating
loops; and focusing means, positioned to receive the
spatially-scrambled optical signal that is generated and direct the
spatially-scrambled optical signal outward from the FSOCS
transmitter as a spatially-scrambled optical beam.
18. The FSOCS transmitter of claim 17, wherein the means for
generating a spatially-scrambled optical signal comprises: lasing
means for generating a light signal; and means for converting the
light signal into a mode-scrambled signal.
Description
FIELD OF THE INVENTION
The present invention generally relates to free-space optical
communications systems (FSOCSs), and, more specifically, to a
method and apparatus for increasing laser output power while
maintaining compliance with eye safety standards.
BACKGROUND INFORMATION
With the increasing popularity of wide area networks (WANs), such
as the Internet and/or the World Wide Web, network growth and
traffic has exploded in recent years. Network users continue to
demand faster networks and more access for both businesses and
consumers. As network demands continue to increase, existing
network infrastructures and technologies are reaching their
limits.
An alternative to present day hardwired or fiber network solutions
is the use of wireless optical communications. Wireless optical
communications utilize point-to-point communications through
free-space and therefore do not require the routing of cables or
optical fibers between locations. Thus, wireless optical
communications are also known as free-space or atmospheric optical
communications. For instance, in a FSOCS, a beam of light is
directed through free-space from a transmitter at a first location
to a receiver at a second location. Data or information is encoded
into the beam of light, and therefore, the information is
transmitted through free-space from the first location to the
second location.
Transmission of optical signals through free space poses many
challenges. Notably, atmospheric conditions can greatly degrade
signal strength, and consequently, reduce the maximum link
distances. Also, when launching a single-mode beam from a
free-space optical terminal using conventional means, atmospheric
scintillation and other wavefront distortion cause the beam to
break up into chaotic bright and dark spots. Stated another way,
such beams generally have non-uniform power distributions that vary
on a timescale of milliseconds (corresponding to the transit time
of wind passing through the free-space beam).
In some FSOCS applications, non-uniform power distributions far
from the transmitter tend to undesirably limit the permissible
overall power of the optical signal because the peak possible
irradiance must meet specified eye safety standards. For example,
some FSOCS applications must comply with specified laser
classifications that address eye safety standards, such as the
laser classifications defined by International Electrotechnical
Commission (IEC) International Standard 60825-1: 1993+A1:1997+A2.
To comply with the applicable standard(s), the power of the
transmitted signals must be limited such that the peak possible
irradiance received at a person's eye is maintained below the
specified maximum value.
SUMMARY OF THE INVENTION
According to aspects of the present invention, an apparatus and
method is provided for generating a FSOCS optical signal via a
spatially-extended light source. In one embodiment, the
spatially-extended light source is in the form of a mode-scrambled
optical fiber light distribution and the transmitter employs
single-aperture optics. An optical beam having a "top hat"
intensity profile is produced, which provides several advantages.
One of these advantages is the transmitter power for a given laser
product classification may be increased while still satisfying peak
irradiance limits defined for eye safety.
In one aspect of the invention, a spatially-extended light source
is achieved by employing a laser beam source that directs a laser
optical signal into one end of a first segment of multimode fiber
comprising a graded-index (GI) fiber core. The first segment of
multimode fiber is operatively coupled into a second segment of
multimode fiber comprising a step-index (SI) fiber. As the laser
optical signal passes through the first and second segments of
multimode fiber, the optical signal is converted into a
mode-scrambled optical signal having a substantially filled
numerical aperture. This signal, in turn, is passed through a
collimating lens and directed outward as a mode-scrambled optical
beam.
BRIEF DESCRIPTION OF THE DRAWINGS
Non-limiting and non-exhaustive embodiments of the present
invention are described with reference to the following figures,
wherein like reference numerals refer to like parts throughout the
various views unless otherwise specified.
FIG. 1 is a schematic diagram illustrating the light-ray
characteristics of a conventional FSOCS transmitter that employs a
single point light source;
FIG. 2 is a schematic diagram illustrating the light-ray
characteristics of an FSOCS transmitter that employs a
spatially-extended light source that provides increased laser
power/eye safety capability.
FIG. 3 is a functional block diagram of a FSOCS having a
transmitter that employs a spatially-extended light source,
according to one embodiment of the present invention.
FIG. 4 is a flowchart illustrating a method of increasing laser
power without exceeding the peak irradiance specified in an
applicable eye safety standard, according to an embodiment of the
present invention.
FIG. 5 is a schematic diagram of a mode scrambler in accordance
with a first embodiment of the invention.
FIGS. 6a and 6b are schematic diagrams illustrating details of a
laser beam that is directed toward a free end of a multimode fiber
core, wherein FIG. 6a shows a centerline launch condition, and FIG.
6b shows an offset launch condition.
FIG. 7 is a schematic diagram of an offset mode scrambler in
accordance with a second embodiment of the invention.
FIG. 8 is a schematic diagram of an offset mode scrambler in
accordance with a third embodiment of the invention.
FIG. 9 is a schematic diagram illustrating details of a strain
scrambler employed in the embodiment of FIG. 8.
FIG. 10 shows a cross-section detail of an offset fiber mount and
laser beam source in accordance with one embodiment of the
invention.
FIG. 11 shows a detailed cross-section of a fusion splice used to
couple fiber segments having different core diameters, according to
an embodiment of the present invention.
FIGS. 12a and 12b respectively show a mode-scrambled optical signal
produced by using a prior art mode-scrambling technique that has an
under-filled numerical aperture, and a mode-scrambled optical
signal produced by an embodiment of the present invention in which
the numerical aperture is substantially filled according to an
embodiment of the present invention.
FIG. 13 is an isometric view of an exemplary FSOCS transceiver that
employs a spatially-extended light source in accordance with one
embodiment of the invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of an apparatus and method for generating
mode-scrambled optical signals are described herein. In the
following description, numerous specific details are disclosed to
provide a thorough understanding of embodiments of the invention.
One skilled in the relevant art will recognize, however, that the
invention can be practiced without one or more of the specific
details, or with other methods, components, materials, etc. In
other instances, well-known structures, materials, or operations
are not shown or described in detail to avoid obscuring aspects of
the invention.
Reference throughout this specification to "one embodiment" or "an
embodiment" means that a particular feature, structure, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
the appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment. Furthermore, the
particular features, structures, or characteristics may be combined
in any suitable manner in one or more embodiments.
In order to better understand the eye-safety issue, reference is
made to FIG. 1, which illustrates a typical light ray path that
results from "single point" light source and collimating optics
commonly employed in conventional FSOCS transmitters. FSOCS systems
work by transmitting modulated laser light toward an optical
receiver. Generally, transmitter and receiver components may be
packaged separately, or may be combined in a transceiver head or
terminal. Focus herein is directed toward the transmit aspect of an
FSOCS transmitter or transceiver.
Ideally, a collimated optical signal 100 illustrated by light rays
100A and 100B would be employed to transmit data via the modulated
laser. Collimated optical signals are analogous to a column of
light, wherein the divergence .THETA. of the light rays is
substantially 0 degrees. To achieve a collimated signal, a point
light source 102 is placed at the focal point 104 of a collimating
lens 106. The focal point 104 is located along the optical axis 108
of collimating lens 106 at a distance f from the primary principal
plane 109 of the lens. Light corresponding to collimated optical
signal 100 that is received by an eye 110 is focused on the eye's
retina 112 via cornea and lens 114. Since the collimated light
signal is emitted from a single point source, the source may be
focused onto a very small area of the retina 112, potentially
causing retinal damage if the maximum permissible exposure (as
specified by the referenced laser safety standard) is exceeded.
In practice, purely collimated optical signals are not used in
terrestrial FSOCS. One reason for this is that it is very difficult
to align transmitters to receivers (since the beam spot area
received at the receiver is so small). Instead, the single point
optical source is moved toward the collimating lens to produce an
output beam with a small amount of divergence. This is illustrated
in FIG. 1 by a light source 102A, which is located on optical path
108 at a defocus distance D away from focal point 102. The
resulting optical signal 116 is defined by light rays 116A and
116B. Neglecting the effects of diffraction, the angle of
divergence, .THETA..sub.1, is approximately equal to DA/f.sup.2
where A is the lens aperture diameter. It is noted that angles and
displacements are exaggerated in several of the drawing Figures
herein for clarity--the actual angles of divergence are
significantly smaller.
Although the optical signal now has a small amount of divergence,
the optical source is still a single point source that can be
sensed as such by the human eye, resulting in a similar potential
for retinal damage. The net result is that divergent optical signal
116 still produces high peak irradiation. Furthermore, the location
of the peak irradiation is unpredictable, as follows.
There is a dichotomy in FSOCS signal design. Designers often feel
the need to produce high quality optical systems with "high
quality" optical beams, which usually implies a Gaussian spatial
distribution. Gaussian beams propagate in vacuum as eigenmodes,
meaning their characteristic shape does not change with distance;
these "ideal" beams are easily described in theory, but in practice
the quality of the beam degrades as soon as it exits a finite
transmitter. A beautiful Gaussian-like beam can quickly degenerate
into a multimode spatial profile of the worst variety, with deep
intensity nulls and high peaks that may vary in location and time
as a function of air turbulence, density fluctuations and other
phenomena, such as lens or window aberrations. An example of a
deteriorated Gaussian beam profile is shown in FIG. 12a and
discussed below.
Since it is not possible to prevent a "quality" beam from degrading
in the atmosphere, the immediate question is this: can we purposely
"scramble" the beam, complicating it to the point that atmospheric
phenomena have a minimal influence on how the beam propagates?
There are a number of ways of viewing this problem. One may view
the vacuum eigenmodes as the most "orderly" solution, with one
allowed state for a given beam size and divergence. However, the
number of possible "disorderly" states is many times greater than
one; it is much more likely that light will scatter or diffract
from the orderly state into one of these disordered states. Once
the light is in this disordered state, it will tend to scatter into
another disordered state. The disordered nature of this model is
analogous to the entropy model used in thermodynamics and other
phenomena. It is highly unlikely that a disordered beam will
scatter back into the Gaussian vacuum eigenmodes. The best beam to
transmit in the atmosphere is the one that is disordered enough
such that it always scatters into a new beam with equivalent
entropy. Nevertheless, the beam must not be so disordered that it
cannot be used in a practical optical link.
Ideally, the highest entropy beam would come from an extended,
omnidirectional white light source, but such a source is not
practical. One practical method for producing a high-entropy beam
is to use a multimode, or spatially-extended light source created
with light from a laser.
What is an extended light source? It first helps to understand
"single mode" sources that are most often used for optical
transmissions. A good example of a single mode source is light
originating from a single mode fiber or from a single spatial mode
diode laser. These sources are often referred to as "nearly" point
sources. Beams derived from these point sources have a
Gaussian-like intensity pattern. They can be tightly focused with a
good lens (or the human eye). An extended source, on the other
hand, is like many millions of single mode sources arrayed next to
each other. An example of an extended source are light emitting
diodes or light from a large core optical fiber, where the fiber
mode power distribution is relatively uniform.
The present invention addresses the foregoing peak irradiance eye
safety consideration by employing a spatially-extended light
source, also referred to herein as an "extended source." The
extended source is embodied in the form of a multi-mode fiber
within which a large collection of separate modes are excited. The
resulting mode-scrambled irradiance distribution yields an improved
FSOCS transmission source compared to the conventional point
source. These improvements include increasing the permissible
transmitter power for a given laser product classification,
reducing received power fluctuations under severe scintillation
conditions, and other conditions and characteristics described
below.
In one embodiment, light from a high modulation rate light source
is spatially scrambled by appropriate means in large core optical
fiber. The appropriate means may include a combination of fiber
sizes and numerical apertures (NA), light launch conditions,
scrambling elements, such as diffractive optical elements or
lenslet arrays or random surface profile diffusers. The purpose of
the scrambling is two-fold: (1) to increase the number of excited
modes to complicate the optical beam enough that the effective
spatial coherence is reduced, without adversely affecting the data
rate, and (2) to "fill" the NA of the fiber to provide a consistent
and repeatable optical source. "Filling" the NA of the fiber also
increases the number of modes and provides a more uniform extended
source.
The effect of this mode-scrambled light source is that light from
what is effectively a point source (the laser) is converted into an
extended source, one that emits light over a much broader aperture;
the optical power, instead of arising from a single point source,
is now distributed across an area that is considerably larger. The
transmitted radiance (in units of power per unit area per unit
solid angle) drops by the ratio of the areas of the diffraction
limited "point" source to the fiber core area, so the radiance may
drop by orders of magnitude. One result of this is that the minimum
spot size of the extended source on the eye's retina is larger than
that for a point source, and the peak irradiance (in units of power
per unit area) within the focused spot is reduced.
For wavelengths in the range of 400 to 1400 nm, the maximum
permissible exposure limit is primarily determined by irradiance
and spot size at the retina. When the apparent angular extent,
i.e., diameter, of an extended source exceeds 1.5 milli-radians
(determined either with direct viewing conditions or by magnified
viewing conditions depending on the specific laser classification)
the permissible maximum power goes up as a result, allowing more
power out of the aperture without compromising safety. A
well-scrambled optical fiber extended source also approaches a
top-hat shaped distribution, both in the irradiance at the tip of
the emitting fiber core and in the far field irradiance pattern
from the fiber emitter where a collimating lens would typically be
placed, so the eye safety level increases as well from this uniform
power distribution (as compared with a Gaussian-like power
distribution).
An extended source transmitter can be designed such that the
divergence is minimized by locating the extended source in the
focal plane of a single-aperture optical transmitter system. For
example, FIG. 2 shows an embodiment in which a spatially-extended
light source 200 is located in the focal plane 202 of a lens 206.
The center of spatially-extended light source 200 is coincident
with optical axis 208 of lens 206. The spot size of the extended
source, in terms of its diameter, is d. The resultant diverging
angle .THETA..sub.2, is equal to d/f.
Locating the extended source in the focal plane is preferable to
locating it along the optical axis 208 such that divergence is
determined mainly by defocus, such as was the case in FIG. 1. The
reason for this is because for a given divergence, the apparent
angular subtense of the source (that accounts for the eye's ability
to focus at different ranges) is maximized. Placing the fiber tip
in the focal plane of the transmitter lens (e.g., lens 206) further
simplifies the overall system design, since the effective source
location is at infinity for all possible distances between the
FSOCS terminals in a link. This can also simplify manufacturing
since the divergence is less sensitive to the longitudinal
placement of the fiber tip. This top hat irradiance distribution
also means that the received power will fluctuate less as a result
of vibrations on the transmitter.
One advantage of launching a spatially-extended light source signal
comprising a mode-scrambled signal with a
substantially-uniformly-filled numerical aperture is that a more
powerful signal can be transmitted for a given classification of
laser product. For example, the maximum amount of power used for a
given optical signal due to eye safety concerns will generally be
limited as a function of the power collected within a specific
measurement aperture, (e.g. 7 mm diameter), rather than the
integrated intensity of the signal (i.e., total signal power).
Accordingly, the maximum power used for conventional signals for
which the irradiance is not uniform across the emitting aperture
will be limited by their peak intensities, which are often much
more pronounced (relative to an average intensity) than that found
in the top-hat profile produced by embodiments of the present
invention, as presented below. As a result, the present invention
enables more powerful optical signals to be transmitted, while
still adhering to eye safety limitations.
FIG. 3 is a block diagram illustrating the primary components of an
FSOCS 100 including an FSOCS transmitter with increased laser
power/eye safety capability, according to one embodiment of the
present invention. Under well-known practices, an FSO signal 102 is
transmitted from a FSOCS transmitter 104 through the atmosphere and
received at an FSOCS receiver 106. FSOCS transmitter 104 includes a
power controller 108, a data modulator 110, a spatially-extended
light source 112 including a laser 114, and output optics 116.
With reference to the flowchart of FIG. 4, FSOCS transmitter 104 is
operated in the following manner to produce an optical signal
having maximum power while still meeting an applicable eye safety
peak irradiance limit. First, in a block 400, FSOCS signal 102 is
generated through use of spatially-extended light source 112. In a
block 402, a peak irradiance of the FSOCS signal 102 is determined.
For example, the peak irradiance may be measured near the output
optics 116. Photonic measurement devices, such as photometers and
the like, may be used to determine the peak irradiance of the
signal. The power supplied from power controller 108 to laser 114
is then adjusted in a block 404 so that the peak irradiance of
FSOCS signal 102 remains below the threshold eye safety limit for
the applicable laser class.
A laser light source can be converted into an extended source by a
number of means. The simplest method is to insert a diffuser screen
into the beam, but his method does not produce the type of source
that is particularly useful. A better method is to fully populate
modes in large core optical fiber. Fiber has the advantage that
light emitted from it is constrained in angle to be within the
fiber's NA so that the fiber may be matched to an optical
transmitter telescope without much loss of total power. The fiber
extended source, if made to have (substantially) uniform intensity,
can then be used to produce a "top hat" power distribution from the
transmit aperture, where all of the power from the fiber is
transmitted in an optical beam with a well-defined divergence.
In accordance with further aspects of the invention, a mode
scrambler may be employed to convert a laser-generated signal into
a spatially-extended signal. For example, a mode scrambler 510 in
accordance with one embodiment of the invention is shown in FIG. 5.
In this embodiment, mode scrambler 510 includes a laser beam source
512 that directs a light beam 514 toward an input end of a segment
of graded index (GI) multimode optical fiber 516. As used herein,
the term "fiber" will generally refer to optical fiber, and the
terms fiber and optical fiber are used interchangeably. GI
multimode optical fiber segment 516 is coupled to a segment of
step-index (SI) multimode optical fiber 518 via a fiber coupler
520. In one embodiment, GI multimode optical fiber segment 516
comprises a 62.5 micrometer (micron or .mu.m) core, while SI
multimode optical fiber segment 518 comprises a 200 .mu.m core.
As the light beam 514 passes through GI multimode optical fiber
segment 516, it begins to be scrambled into a plurality of modes.
Upon passing through a GI multimode optical fiber-to-SI multimode
optical fiber interface connector 520 and passing through SI
multimode optical fiber segment 518, the original laser optical
signal is emitted from a free end 522 of the SI multimode optical
fiber segment as a mode-scrambled laser output 524.
In general, laser beam source 512 may comprise one of many types of
laser beam sources that can produce a modulated laser beam. For
example, laser beam source 512 includes a laser diode 526 mounted
to a first face 528 of a housing 530. Laser diode 526 emits laser
light 532, which is received by a focusing optical component 534.
In one embodiment, focusing optical component 534 comprises a
single optical lens. In another embodiment, as illustrated below in
FIGS. 9 and 10, focusing optical component 534 comprises a set of
optical lenses. In either case, the single or set of optical lenses
is/are operatively coupled to housing 530 such that focusing
optical component 534 is held in a fixed relationship to laser
diode 526.
As further shown in FIGS. 6a and 6b, laser diode 526 and focusing
optical component 534 are configured in a manner that results in a
light beam 514 being directed toward a focal point F.sub.p that is
substantially coincident with the fiber core 536 of the end face of
GI multimode optical fiber segment 516. In the embodiment of FIG.
6a, the focal point F.sub.p of light beam 514 is substantially
coincident with a centerline C of fiber core 536. Conversely, in
the embodiment of FIG. 6b, the focal point F.sub.p of light beam
514 is offset from centerline C by an offset distance O. This
generates an offset-launched optical signal that is received at the
end of fiber core 536. In one embodiment, focusing optical
component 534 is configured such that a convergence angle .alpha.
of light beam 514 substantially matches the numerical aperture of
fiber core 536.
As further depicted in FIGS. 6a and 6b, fiber core 536 is
surrounded by cladding 538. In typical fibers, the fiber cladding
is generally surrounded by a protective jacket, made of materials
such as polymers. For illustrative purposes, the fiber core,
cladding, and protective jacket are shown as a single structure in
several of the Figures contained herein for clarity.
Returning to FIG. 5, in one embodiment, an end portion of GI
multimode optical fiber segment 516 is held in a fiber mount 540,
which is mounted to an end face 542 of housing 530 such that focal
point F.sub.p is substantially coincident with the end of fiber
core 536. In general, any suitable means for fixedly mounting the
end of fiber core 536 such that it is substantially coincident with
focal point F.sub.p may be used.
A mode scrambler 700 in accordance with another embodiment of the
invention is shown in FIG. 7. In this configuration, light beam 514
is directed into fiber core 536 such that an acute angle .theta. is
formed between respective centerlines 545 and 546 of the light beam
and end portion of fiber core 536. The remaining components of mode
scrambler 700 are substantially similar to like-numbered components
discussed above with reference to mode scrambler 510. In the
embodiment illustrated in FIG. 7, an end face 542A of a housing
512A is angled relative to centerline 545 such that it forms an
angle of 90.degree.--.theta. to the first end of GI multimode
optical fiber segment 516. As further illustrated in FIG. 7, angle
.theta. between centerlines 545 and 546 is created upon mounting
fiber mount 540 to end face 542A.
The primary purpose of creating an acute angle between centerlines
545 and 546 is to substantially eliminate any portion of light
impinging on the end of fiber core 536 from being reflected back
toward laser diode 528. Since free space optical signals comprise a
laser beam modulated at very high frequencies, it is desirable to
minimize any signal degradation that might result from the
reflected light. A secondary purpose for this angled fiber launch
is to increase the portion of the fiber numerical aperture that is
filled by light beam 514.
An offset-axis mode scrambler 800 comprising a variation of mode
scrambler 700 embodiment of the invention is illustrated in FIG. 8.
In this configuration, a portion of SI fiber segment 518 is
configured as a series of alternating loops. Further details of the
alternating loops are shown in FIG. 9. In one embodiment, the
alternating loops may be formed by wrapping a portion of SI fiber
segment 518 around a plurality of rods 902 in an alternating
manner. In general, the radius R of the loops should be large
enough to not cause damage to the fiber. In one embodiment, the
rods have a diameter of about 1/2 inch. Additionally, the
horizontal distance D between adjacent rods is generally not
critical.
Details of a fiber mount 1000 that is coupled to a laser beam
source 1002 are shown in FIG. 10. The laser beam source includes a
laser diode 526 that is mounted in a recess 1004 defined in a first
end face 1006 of a housing 1008. In this embodiment, laser light
emitted from laser diode 526 is collimated by a collimating lens
1010 and received by a focusing lens 1012, which directs the laser
light substantially along a centerline 1013 toward a focal point
Fp. An end portion of GI fiber segment 516 is mounted within a
ferrule 1014 having a head portion disposed within a counterbored
hole 1016 defined in fiber mount 1000. Counterbored hole 1016 is
formed such that its centerline (coincident with a centerline 1018
of an end portion of GI fiber segment 516) forms a relative angle
of .theta. between the centerline and a line perpendicular to face
1020 of fiber mount 1000.
In one embodiment, GI multimode fiber segment 516 is coupled to SI
multimode fiber segment 518 using a fusion splice. Details of an
exemplary fusion splice 1100 are illustrated in FIG. 11. As shown
in FIG. 11, in one embodiment, one end of 62.5 .mu.m fiber core
1102 is spliced to one end of a 200 .mu.m fiber core 1104. At the
same time, cladding 1106 surrounding 62.5 .mu.m fiber core 1102 and
cladding 1108 surrounding 200 .mu.m fiber core 1104 are also fused.
The fused portions of the fiber cores and cladding are depicted as
a fusion splice 1110. Prior to fusing the cores and surrounding
cladding, an end portion of jackets 1112 and 1114 surrounding
cladding 1106 and 1108, respectively, is stripped back, and the end
of the fibers are cleaved. Heat is then applied while holding the
ends of the fibers in contact with one another. In one embodiment,
a protection sleeve 1116 may be used to protect the splice and the
exposed cladding. In one embodiment, the protection sleeve
comprises a plastic heat-shrink tube with a metal core 1118.
In general, the fibers in the fusion splice may have their
centerlines co-aligned, as shown in the FIG. 11, or the centers may
be offset. It is further noted that the fusion splice illustrated
in FIG. 11 shows an idealized fusion splice; in actual practice,
there will likely be a discontinuity between the two segments of
fiber.
Additional Advantages of Launching a Mode-scrambled Optical Signal
with a Substantially-filled Numerical Aperture
As discussed above, the spatially-extended light source embodiments
described herein create a mode-scrambled signal with a
substantially filled numerical aperture. The numerical aperture is
basically a measure of the light-gathering ability of the optical
fiber and the ease in coupling light into the optical fiber. The
numerical aperture is defined as the sine of the largest angle an
incident light beam can have for total internal reflection in the
core, and for SI multimode fiber is characterized by:
NA=sin(.theta.)= {square root over
((n.sub.1).sup.2-(n.sub.2).sup.2)}{square root over
((n.sub.1).sup.2-(n.sub.2).sup.2)} where NA is the numerical
aperture, .theta. is the half angle of the incident light beam,
n.sub.1, is the index of refraction for the optical fiber core, and
n.sub.2 is the index of refraction for the optical fiber
cladding.
Light rays launched within the angle specified by the optical
fiber's numerical aperture excite optical fiber modes. The greater
the ratio of core index of refraction to the cladding index of
refraction results in a larger numerical aperture.
Launch conditions corresponding to an under-filled and
substantially filled numerical aperture are illustrated in FIGS.
12a and 12b, respectively. In FIGS. 12a and 12b, optical signals
1202A and 1202B are respectively launched from segments of optical
fiber 1200A and 1200B. As the optical signals impinge upon a
collimating lens 1204, the signals are (substantially) collimated
into respective transmitted signals 1206A and 1206B, which are
received by a FSO terminal (not shown) to complete the link. In
these Figures, the dashed lines illustrate relative intensity
values, wherein the heavier the line, the greater the
intensity.
At the right hand of each figure is an intensity distribution
diagram that depicts the relative power distribution P of the
optical signal vs. angle .THETA. relative to a centerline of the
signal. In practice, the actual intensity distribution comprises a
three-dimensional profile, with the two-dimensional profiles shown
in FIGS. 12a and 12b being for illustrative purposes.
FIG. 12a illustrates two intensity distributions 1208A and 1210.
Intensity distribution 1210 is illustrative of a theoretical
Gaussian profile. As discussed above, the conventional single-point
launch produces a Gaussian-like profile at the launch point (i.e.,
exiting the launch fiber); as the optical signal traverses the
atmosphere and/or passes through optics and windows, uneven optical
effects cause distortion to the Gaussian curve, which are
illustrated in intensity distribution 1208A. Generally, the peak
intensity will be located near the center of the profile, although
the encountered optical effects may cause it to be offset.
In contrast, the signal intensity profile produced by embodiments
of the present invention, as illustrated by an intensity
distribution 1208B, is in the shape of a "top hat," which is a
desirable intensity distribution for optical communication. For
example, one advantage of the "top hat" intensity distribution is
that, for a given safety classification of laser product, it allows
for more energy to be transmitted out of the transmit aperture than
the Gaussian distribution characteristic of a single mode
transmission, or large peak and valley profile common to prior art
mode-scrambled signals.
Another advantage of launching a mode-scrambled signal with a
substantially-filled numerical aperture is that the optical signal
is pre-distorted such that effects such as atmospheric
scintillation and/or window wave front aberration are small
compared to the scrambling generated on the transmitting end. This
means that the light beam power distribution at the receiving
aperture is more homogenous and the intensity fluctuations caused
by atmospheric scintillation and/or window wave front aberration
are practically transparent.
A top hat intensity, extended source distribution is an improvement
over a Gaussian distribution for the additional following
reasons:
(1) The Gaussian vacuum eigenmode can never be allowed to fill the
exit aperture because intermediate field diffraction effects
(Fresnel diffraction) will produce unmanageable diffraction maxima
and minima; the Gaussian mode field diameter must be much less than
the clear aperture of the optical system. Such beams also focus
with high brightness on the retina. In contrast, a top hat beam,
specifically from an extended source, has a certain amount of
natural divergence and can also "fill" the exit aperture without
excessive loss and without concentrating the power in the center of
the aperture. The eye safety power limit is greater as a result of
this combination of filling the aperture and extended source
divergence. The filled aperture distributes the power more evenly,
lowers the radiance, and the extended source divergence reduces the
focused irradiance at the eye's retina. An extended source that has
a nearly top hat shape that fills the exit aperture will greatly
increase the total eye-safe power out of the aperture without
resulting in noticeable Fresnel diffraction effects.
(2) The Gaussian vacuum eigenmode is not an eigenmode of the FSOCS
optical system and is not an appropriate choice. Considering the
entire communication link as the optical system (including air
turbulence, window aberrations, etc.) requires one to recognize
that the Gaussian eigenmodes will never be the appropriate choice.
The practical eigenmode is one that does not significantly change
as it propagates across the link. An extended source produces a
beam that is significantly the same from one end of the link to the
other (provided the link is not excessively long or the transmit
aperture is not excessively small.) This top hat pseudo-eigenmode
is essentially unaltered by atmospheric turbulence or window
aberrations (unless the aberrations are so severe that one can see
the aberrations or turbulence, such as mirage effects, with the
naked eye.)
(3) When the light source is from a single mode fiber, the power
distribution has a bell shape that is approximately Gaussian. This
smooth shape is compromised with any modest number of scratches or
dust on the fiber tip. Alternatively, light directly from a laser
diode facet is elliptical and, from one laser to the next, this
elliptically can vary by several degrees of divergence. An extended
source allows one to build an optical system that does not need to
compensate for the vagaries of these light sources, since
variations between different light sources are lost in the
mode-scrambling. It is therefore possible to make a simpler optical
design and improve the manufacturability of the total FSOCS.
(4) A larger transmit divergence in FSOCS translates into reduced
tracking requirements, but also geometric power loss at the
receiver. While not a complete solution to this problem, increasing
the transmit divergence using extended sources also allows some of
the power loss to be reduced since higher powers are allowed out of
the transmit aperture.
(5) Lowering tolerances on laser sources allows the use of lower
cost lasers and components.
(6) Using large core optical fiber in the extended source allows
the optical head to be de-coupled from the electronics in the
mechanical assembly. This promotes modularity of design, which has
obvious advantages.
An exemplary FSOCS transceiver 1300 that employs spatially-extended
transmitter elements discussed above is shown in FIG. 13. FSOCS
transceiver 1300 employs a binocular configuration including a
transmit optic 1302 and a receive optic 1304. A laser beam source
assembly 1306 generates laser light that is launched into a first
end of an Si fiber segment 1308. After traversing the Si fiber
segment, the light passes through a coupling 1310 that couples the
Si fiber segment to a GI fiber segment 1312. The light then passes
through GI fiber segment 1312, which excites a large number of
modes, resulting in mode-scrambled light 1314 exiting an exit fiber
end 1316 of GI fiber segment 1312. The exit fiber end 1316 is held
by a fiber mount 1318. As mode-scrambled light 1314 impinges on
transmit optic 1302, it is collimated into a mode-scrambled optical
beam 1320, which is transmitted to be received at a receiver optic
1304 on another transceiver (not shown).
In the illustrated embodiment, laser source 1306 includes a laser
(not shown) mounted to a heat sink 1322, which, in turn, is mounted
to a circuit board 1324. Fiber mount 1318 and a fiber mount 1326 in
which the receive end 1327 of a receiver fiber (not shown) are
coupled to a plate 1328. Transmit and receive optics 1302 and 1304
are coupled to a plate 1330. Plates 1328 and 1330 are coupled via a
rear cross-plate 1332 and mid and front cross plates (both removed
for clarity), thereby forming a frame assembly 1334.
All of the illustrated components of FSOCS transceiver 1300 are
mounted within a housing, which is not shown for clarity. Under a
typical use, the housing is mounted to a support member, or is
otherwise operatively coupled to a building member (e.g., wall or
floor). Typically, respective FSOCS are mounted in offices of
buildings that are within line-of-sight of one another, wherein the
optical signals are transmitted through building windows.
Optionally, one or both of the transceivers may be mounted on the
exterior of a building.
In the foregoing detailed description, the method and apparatus of
the present invention have been described with reference to
specific exemplary embodiments thereof. It will, however, be
evident that various modifications and changes may be made thereto
without departing from the broader spirit and scope of the present
invention. The present specification and Figures are accordingly to
be regarded as illustrative rather than restrictive. Furthermore,
it is not intended that the scope of the invention in any way be
limited by the above description, but instead be determined
entirely by reference to the claims that follow.
* * * * *